This invention relates to a fuel cutoff control method for an engine and particularly to a fuel cutoff control method during deceleration for an automobile engine.
In a conventional motorcycle or the like with a fuel injection engine mounted, engine operating conditions are determined based on load and engine speed. When it has been determined to be unnecessary to supply fuel because the engine is in a decelerating state, the supply of fuel is cut off and a fuel injection amount is reduced to 0. Reducing the fuel injection amount to 0 during deceleration provides a decelerating effect of engine braking, as well as improved fuel efficiency and cleaner exhaust gas.
Such a configuration is also used as a means for cleaning exhaust gas that a secondary air introducing pipe is connected to an exhaust pipe, and exhaust gas is recombusted with secondary air introduced to the exhaust pipe by utilizing the pulse of exhaust gas pressure. An air cut valve is provided on the secondary air introducing pipe. The air cut valve opens at high engine speed with the throttle opened during normal driving or acceleration to introduce secondary air, while closing at low engine speed with the throttle closed during deceleration to cut off the secondary air.
In a vehicle including this type of secondary air introducing system, during deceleration, the supply of secondary air to the exhaust pipe is cut off, and the supply of fuel is also cut off to reduce the fuel injection amount to 0 as described above.
However, in the case that the engine shifts from a normal running state to a decelerating state with the throttle being closed, if fuel is cut off simultaneously with the start of deceleration, fuel adhered to an intake pipe wall flows into a cylinder immediately after fuel is started being cut off. Since the adhered fuel is too lean to combust alone in the cylinder, it is exhausted into the exhaust pipe as unburned gas. In the case of a vehicle including the secondary air introducing system, the unburned gas which flows into the exhaust pipe reacts with secondary air to combust, thereby causing afterburning. The afterburning affects the engine itself as well as causing noise.
In order to prevent the afterburning, the foregoing air cut valve is provided to cut off the supply of the secondary air when the intake pipe pressure has been lowered to a specified value or below.
The air cut valve, however, operates with a delay, so that even if it is driven to be closed simultaneously with a fuel cutoff command, no afterburning can be completely prevented which occurs during an early stage after fuel has been cut off.
In view of the foregoing, an advantage of this invention is to provide a fuel cutoff control method for an engine including a secondary air introducing system, capable of preventing afterburning in an exhaust pipe at the time of fuel being cut off in a decelerating state of the engine.
In order to achieve the foregoing advantage, the invention provides a fuel cutoff control method during deceleration of an engine including a secondary air introducing pipe connected to an exhaust pipe, and an air cut valve provided on the secondary air introducing pipe, the method comprising a determination step of determining under predetermined conditions whether or not the engine is in a decelerating state, in which in the decelerating state, a greater amount of fuel than a normally required amount of fuel flow is supplied during a predetermined period of time after the start of deceleration, and then fuel is cut off after the predetermined period of time has elapsed.
In this configuration, even if it has been determined that fuel be cut off in a decelerating state of the engine, the fuel injection amount is not immediately reduced to 0. A required amount of fuel flow computed according to a normal program is multiplied by an increasing coefficient and an increased amount of injected fuel is supplied during a predetermined period of time until the air cut valve of the secondary air introducing pipe is closed. The fuel injection amount is reduced to 0 after the predetermined period of time has elapsed and the air cut valve has been closed. Thereby at the start of deceleration, when a throttle is closed and the fuel adhered to the intake pipe wall flows into the cylinder due to increased negative intake pressure, the adhered fuel completely combusts in the cylinder together with the additional fuel injected from the injector (the adhered fuel is too lean to combust alone). Therefore, nounburned gas flows into the exhaust pipe. Afterburning caused by reaction of the unburned gas and secondary air in the exhaust pipe is thus prevented.
Incidentally, the predetermined period of time may be set as the elapse of a predetermined time through time control or as predetermined engine cycles in the case that fuel cutoff control judgment is made per stroke (cycle).
In a preferred embodiment, when it has been determined at the determination step that the engine is not in the decelerating state, if the engine is regaining speed back from the decelerating state, a less amount of fuel than normally required is supplied.
In this configuration, when the engine returns to a normal running state back from the decelerating state while the throttle being reopened, a required amount of fuel flow computed according to a normal program is multiplied by a reducing coefficient and a reduced amount of required injected fuel is supplied. This prevents a sharp increase in engine speed caused by a sharp increase in the fuel injection amount and maintains a stable running state of a vehicle.
In another preferred embodiment, the fuel cutoff control method further comprises as the predetermined conditions: a step of determining the time elapsed since an engine start; and a step of determining cooling water temperature.
In this configuration, an intake air amount is calculated based on intake pipe pressure data detected as computing data at an appropriate crank angle. The intake air amount and engine speed can appropriately determine the operating conditions of the engine. Further, fuel cutoff control is prevented at an engine start and when cooling water temperature is low so that the stable operating conditions of the engine can be maintained.